A process has already been proposed in Swiss patent No. CH 580 496 for the production of a pile fabric wherein a sheet of parallel threads is conformed in succession in rows of loops fastened progressively to the surface of a support by causing the material of the threads and of the support to melt locally in their zone of contact by pinching them between a bearing surface and a sonotrode set into acoustic vibration at a frequency and amplitude which can cause the melting of the thermoplastic material. According to this process, the sheet of threads is pleated using a mechanism having an alternating motion superimposed on the defilading motion of the support. This mechanism comprises a matrix provided with a multiplicity of tubes guiding the threads, which are aligned in a crosswise direction with respect to the direction of defilading motion of the support and which are mounted so as to move in a back and forth motion in a plane perpendicular to that of the relative motion between the welding sonotrode and a welding bearing surface so as to form a series of successive parallel loops mounted on the support the bases of which are welded progressively onto the surface of this support in the zones thereof adjacent to that support.
One of the main drawbacks of this system is the nature of the alignment of the rows of loops on the support which is not desirable in the case of a rug because it gives rise to less satisfactory covering power of the support and makes the junctions more difficult to effect because if the rows are not placed perfectly in line with one another, the junction line becomes extremely distinct. This invention aims at correcting, at least partially, the above mentioned drawbacks.